1. Field of the Invention
The invention relates to an angle measuring device, and more specifically, to an absolute-type disk capacitive sensor measuring device for absolute angular position measurement.
2. Description of the Related Art
Conventional capacitive-type measuring devices for angular position measurement adopt variable-area capacitive sensors. By detecting the capacitance change of a movable pole piece relative to a fixed pole piece, angle variation can be measured. Conventional capacitive measuring devices include two types: incremental and absolute. Owing to the limitation on the capacitance amount, conventional absolute-type capacitive measuring devices can only be used to measure absolute small angle position, and thus their application is limited. Wide-angle measurements can be realized using incremental-type capacitive sensors by continuously reading a periodically-alternating phase signal of a grid coupling.
However, incremental-type capacitive sensors have the following drawbacks:
a) The periodically-alternating phase signal is read continuously, and the device is always calculating. If the movable member moves relatively fast, the phase signal will also change at a high speed, and the calculating response speed must be increased to match the phase signal change frequency.
b) To calibrate a measurement, the movable member must be re-zeroed. Calculation errors propagate and multiply.
c) If the power is turned off, the reference position of the movable member must be reset before further use.
To overcome the drawbacks of the incremental-type capacitive sensors, a measuring device for absolute position measurement is disclosed, e.g., in U.S. Pat. No. 4,420,754, and China Pat. Nos. CN89106051, CN93107991, and CN93117701. This measuring device realizes measurement of absolute position displacement by detecting multiple different periodic phase signals generated by the movement of movable members. However, the multiple different periodic phase signals are highly correlated and are all based on the measuring phase difference between a rough division signal and a fine division signal. In addition, the process of calculating the fine phase signal within the rough phase signal periodicity is required in order to calculate the absolute position. The phase signal can then be integrated by applying logical operation suitable for different periodic phase signals, and combining with complicated operation procedure, and measuring each phase periodicity. However, it is complicated to apply synchronous modulation control for calculating the counting pulse according to demand. In order to increase the measurement precision, three different signal patterns, rough pattern, medium pattern, and fine pattern, can be used. With this method, the data processing process is complicated, and the data display frequency can not always match the fast movement steps of the movable members. If higher data processing speed is required, the size of the data processing device must be increased. Therefore, the conventional method cannot be applied in a manual-type measuring device.
China Pat. No. CN03103875 discloses another absolute position measuring device with two sets of rotary encoders rotating relative to each other. The absolute position of the movable member is calculated according to the phase difference between different phase signals. However, owing to the too high requirement on the grid position and the complexity of the signal processing circuit or the device structure, the device cannot be manufactured economically and reliably and thus, does not have a wide applicability.